This document discusses chromosome structure and organization. It begins by listing components of chromosomes like centromeres, telomeres, and origins of replication. It then describes how genes are organized between centromeres and telomeres in eukaryotes. Genes can be relatively short with few introns in lower eukaryotes, and longer with many introns in higher eukaryotes. The document also discusses the differences between chromosomes and chromatin, homologous chromosome pairs, autosomes and sex chromosomes, and types of heterochromatin and euchromatin.

1. Chromosome structure

2. Centromere
Kinetochore proteins
Origin of replication
Origin of replication
Origin of replication
Origin of replication
Telomere
Telomere
Genes
Repetitive sequences

3. Chromosome Organization
• Genes located between centromere & telomeres
– hundreds to thousands of genes
– lower eukaryotes (i.e. yeast)
• Genes are relatively small
• Very few introns
– higher eukaryotes (i.e. mammals)
• Genes are long
• Have many introns
• Non-gene sequences
– Repetitive DNA
• Telomere
• Centromere
• Satellite

4. Chromosomes vs. Chromatin
Chromosomes
• Tightly packaged DNA
• Found only during cell
division
• DNA is not being used for
macromolecule synthesis
Chromatin
• Unwound DNA
• Found throughout
Interphase
• DNA is being used for
macromolecule synthesis

5. Homologous Chromosome Pairs
• Eukaryote chromosomes come in pairs (homologues)
• Normal humans have 46 chromosomes in 23 pairs
• One chromosome of each pair comes from an individual’s
mother, the other comes from their father
• Homologous Chromosomes (chromosome pairs) carry genes
that control the same Characters, e.g., eye color, blood type,
flower color, height, etc.
• Homologous chromosomes have nearly identical structure,
banding patterns, and nucleotide sequences
• Locus: Physical site on chromosomes where given gene is
located
• Allele: Different forms of the same gene, e.g., A, B, or O
blood-type alleles

6. Autosomes & Sex Chromosomes
Autosomes: Found in both males and females
• In humans there are 22 pairs of autosomes
• Autosomes that are the same size (& structure) are called
homologues
Sex Chromosomes: Determine an individual’s gender
• One pair of chromosomes (X and Y)
• The X and Y chromosomes are not homologous
• The X chromosome is much larger than the Y chromosome
and contains many genes
• The Y chromosome has only a small number of genes
• In humans and other mammals females are XX and males
are XY

11. • Lesson 2 - Chromosome structure
• The DNA compaction problem
• The nucleosome histones (H2A, H2B, H3, H4)
• The histone octamere
• Histone H1 the linker histone
• Higher order compactions
• Chromatin loops and scaffolds (SAR)
• Non histone chromatin proteins
• Heterochromatin and euchromatin
• Chromosome G and R bands
• Centromere

12. • Take 4 meters of DNA (string) and compact them
into a ball of 10µM. Now 10µM are 1/100 of a
mm and a bit small to imagine – so now walk from
here to the main entrance let say 400 meters and
try to compact it all into 1 mm.

18. • Histone proteins
basic (+ charged
lysine & arginine)
amino acids that
bind DNA backbone
• Four core histones in nucleosome
– Two of each of H2A, H2B, H3 & H4
• Fifth histone, H1 is the linker histone
Figure 10.14
Nucleosomes

19. Figure 10.14
Nucleosomes

20. Figure 10.21

21. 10-64
Figure 10.21
Compaction level
in euchromatin
Compaction level
in heterochromatin
During interphase
most chromosomal
regions are
euchromatic

24.  Compaction level of interphase chromosomes is
not uniform
 Euchromatin
 Less condensed regions of chromosomes
 Transcriptionally active
 Regions where 30 nm fiber forms radial loop domains
 Heterochromatin
 Tightly compacted regions of chromosomes
 Transcriptionally inactive (in general)
 Radial loop domains compacted even further
Heterochromatin vs Euchromatin

25. Types of Heterochromatin
Figure 10.20
• Constitutive heterochromatin
– Always heterochromatic
– Permanently inactive with regard to transcription
• Facultative heterochromatin
– Regions that can interconvert between euchromatin and
heterochromatin
– Example: Barr body